Recently, ultrahigh-speed data communications techniques, e.g., an optical communications technique using an optical fiber, have advanced rapidly, but demand for transmission of a growing amount of data keeps getting stronger, too. To transmit a larger amount of data, it is necessary to implement a broadband amplification system operating at a wider frequency bandwidth. For this purpose, an active element capable of stably operating in an ultrahigh frequency band has to be developed. Moreover, it is also important to know how to install a newly developed element and others in a broadband amplifier circuit, i.e., to improve the way to design a broadband amplifier circuit in order to make the newly developed elements function effectively. The front-end of an optical communication receiver includes a photodetector, which converts an optical signal into an electrical current signal, and a preamplifier for extending a frequency bandwidth.
One of key components in the broadband amplification system is the preamplifier. A common source transimpedance amplifier (TIA) is usually used as the preamplifier.
FIG. 1 shows a circuit diagram of a conventional common source TIA 100, which includes a plurality of transistors 10, 20, 24 and 30, a multiplicity of resistors 12, 14, 18, 22, 26, 28 and 32 and a power supply 16.
The conventional common source TIA 100 amplifies a current signal inputted thereto by means of four-part transistors 10, 20, 24 and 30, thereby outputting a voltage signal. That is, the transistor 10, a first amplifying transistor, basically amplifies the current signal and the transistor 20, a first buffer transistor, buffers the amplified signal. Thereafter, the transistor 24, a second amplifying transistor, secondarily amplifies the buffered signal from the first buffer transistor 20 and then the transistor 30, a second buffer transistor, buffers the secondarily amplified signal.
A wide frequency bandwidth is one of the most important characteristics to be considered in designing a high-speed optical communications system, especially, TIA. However, it is not easy to extend a frequency bandwidth of the conventional common source TIA as much as required by the optical communications system. Accordingly, many schemes, e.g., gain peaking techniques using passive devices of inductors or capacitors, have been proposed.
A shunt inductive-peaking scheme, one of the most frequently used techniques, connects an inductor to a drain of the first and/or the second amplifying transistor in the conventional common source TIA 100, resulting in a resonance with parasitic capacitances. Although the shunt inductive-peaking scheme extends the frequency bandwidth, stray capacitances of on-chip-inductor often cause a bad influence upon bandwidth increment. The shunt inductive-peaking scheme has further problem in that self-resonant frequency (SRF) and Q-value of the inductor greatly limit on high-frequency applications.
Another method for wideband design is a capacitive-peaking scheme, in which a capacitor is coupled to the resistor 22, in parallel at the source of the first buffer transistor 20 of the conventional common source TIA 100. In the result, an extra pole is added to a transfer function of the conventional common source TIA 100 so that the frequency bandwidth may be extended. However, this method also has serious problems due to the variation of capacitance caused by parasitic pad capacitance and process variation.
Therefore, it is desirable to develop a TIA capable of extending the frequency bandwidth without regard to SRF and Q-value of an inductor and simultaneously less sensitive to parasitic pad capacitance and process variation.